As shown in FIG. 25, an conventional gas engine of the pre-chamber type includes a main chamber 10 (main combustion chamber) and a pre-chamber 12 connected thereto. Lean mixture of fuel and air is supplied through an intake port 14. Rich mixture of fuel and air is formed in the pre-chamber 12 by the fuel gas supplied through a gas pipe 18. A spark plug 20 initially ignites the rich mixture in the pre-chamber 12 to cause combustion in the main chamber 10 so as to achieve low fuel consumption and clean exhaust gas.
15 is a gas regulator, 16 is a throttle valve, 17 is a mixer, 19 is an igniter, 21 is a lubricating oil pipe, 23 is a pump, 25 is an intake valve, 26 is an exhaust valve. A gas pipe 18 is provided with a check valve 24, which is disclosed in the Japanese patent application No. 63-37946 by the applicant. The check valve 24 is operable to be opened by a negative pressure in the chamber 12 and closed by the positive chamber (FIG. 26).
If the engine as described above is large in sizes, the mixture flows in the pre-chamber 12 at excessively high velocity, which deteriorates ignitability in the chamber 12, as stated in the Japanese Utility publication No. 55-14749.
In FIG. 26, the pre-chamber 12 is formed in a body 125 which is screwed into a cylinder head 126 with a packing 127 therebetween. The top end of body 125 is welded to the sleeve 128 to which the plug 20 and the check valve 24 are attached.
The upper portion of the check valve 24 is located in a space 130 connected to the gas pipe 18. The lower portion of the valve 24 is disposed in a space 131 connected to the pre-chamber 12. Thus, the check valve 24 controls the flow of the gas from the gas pipe 18 into the pre-chamber 12 through the space 131.
As shown in FIG. 27, the check valve 24 comprises a valve holder 132, a valve guide 133, a valve stem 134 slidably fitted in the guide 133, a valve body 135 and a valve spring 136. The negative pressure applied to the valve body 135 in the intake stroke descends it away from a valve seat 137 against the elastic force by the spring 136, and thus the valve 24 opens. The upper space 130 is formed in a cap 139, and a lower space 138 is formed in the valve guide 133.
In the prior art structures, when the pressure in the space 130 decreases, it may become difficult to supply the gas to the chamber 12.
Further, a difference between the pressures in the spaces 130 and 138 may restrain the opening operation of the valve body 135, and thus causes a delay thereof, which results in unpreferable opening characteristics. The Japanese patent publication 62-331 relates to this matter.
In order to maintain an ideal mixture ratio, it is necessary to control and vary the supply pressure of the gas or the mixture to the pre-chamber in accordance with the load and engine speed.
However, there has been no method for effectively control the supply pressure to the pre-chamber.
For gas engines of three-way catalyst type without a pre-chamber, there has already been mixture ratio control apparatus, as disclosed in the Japanese laid-open publication No. 61-138840, in which a gas supply pipe and an intake pipe are connected, without through a mixer, by a bypass passage, of which opening degree is controlled by a control device.
This conventional apparatus is however of a type, in which a flow rate of the fuel gas in the bypass passage is controlled, so that precise control is impossible. Since the volume percentage of the fuel gas in the mixture is remarkably small, even if the gas flow rate changes to a small extent, it causes the change the air/fuel ratio to a large extent. In the catalyzer type, the mixture is rich and has a volume ratio, e.g., of 11, so that the mixture ratio will not exceed the combustion limit even by a rough control. However, in the lean mixture combustion type, in which the mixture has a volume ratio of about 22, the precise control is required to maintain the ratio within the combustion limit because the flow rate of the gas rapidly affects the mixture ratio of the lean mixture. Therefore, the conventional control device can not be used in the gas engine of the lean burn type. Further, the conventional device can not perform the control of the ratio in accordance with the load and engine speed.
The inventors have devised the control apparatus for the lean mixture combustion engine, in which a relief passage 364 is connected to a boost pressure compensation pipe 363, which functions to introduce a pressure in an air supply passage 361 to a gas regulator 362. In the relief passage 364, there are provided a flow control valve 365 and a shut-off valve 366, both of which are electrically controlled by a control device 377 receiving signals from a boost sensor 370, a rotation speed sensor 374 and a lean-burn sensor 376. The sensor 370 detects a pressure in a portion downstream the throttle 369 in an intake pipe 368. The sensor 374 detects teeth of a gear 377 on a crank shaft to produce output pulse signals. The sensor 376 detects residual oxygen concentration in the exhaust gas. In the control device 377 consisting of micro-computer and others, the optimum or ideal mixture ratio in relation to the engine load and speed is pre-memorized as a map. The optimum mixture ratio is read out from the map in accordance with the engine speed and load resulting from the processing of the signals from the sensors 370 and 374. The actual ratio is obtained from the signal from the sensor 376. The control valve 365 is controlled so that the actual ratio may have the ideal value. In the controlling operation, the flow of the air relieved through the passage 364 changes, and thus the air pressur introduced from the passage 363 to the regulator 362 changes, so that the mixture ratio varies to the optimum value. In an accelerating operation, the control unit 377 controls the shut-off valve 366 to close, so that the supply gas pressure increases, resulting in the rich mixture. In FIG. 28, 371 is a cylinder, 372 is a piston 372, 379 is a gas supply pipe, 380 is a mixer, 382 is a spark plug, 383 is an intake valve, and 384 is an exhaust valve.
Although the device described above is superior to the previously stated device, it controls the gas supply pressure, so that the change of the opening degree of the valve 365 causes rapid change in the mixture ratio, and thus the sufficiently precise control is impossible.